I really think promise rings are just a bad idea overall, but putting that aside: if you and he have never talked about getting
him a ring, I wouldn't do it until you do. Many guys don't -want- any jewelry (and only begrudgingly wear a wedding ring

). While a promise ring is a nice and fluffy idea full of rainbows and ponies, you really should
talk to him first, please do not make this a 'surprise! promise ring!' idea because he may be cringing on the inside if you do even if he plasters a smile on his face on the outside.
The fact that you have no idea what kind of ring to get him suggests you really have no idea if he even wants a ring because you've never talked about it. Ask him if he'd even
wear a ring and find out what his tastes are. Guys rings come in many different shapes and sizes and even "just a band" can come in a variety of flavors.
And if you -are- going to go this route, don't make it a cheap silver-plated crappy ring, put some money into it, because many guys (since they don't wear a lot of metals on their hands) often find they have a reaction to anything below a good quality gold, my husband has this problem with cheap jewelry, he can only wear good quality stuff. Other types of rings you could look into are titanium, platinum, and tongueston (spelling may be wrong, sorry, too lazy to look it up at the moment

), or something in a solid silver (no silver-plated stuff).
Even though it's not "the big ring", don't cheap out on it. But at the end of the day, I personally will always recommend against promise rings for both guys or girls. Wait until you guys are ready to call an engagement, then get rings for the both of you (something my husband and I did)